Incremental Nonlinear Normal-Mode Initialization

Abstract

An incremental nonlinear normal-mode initialization procedure developed within the National Meteorological Center (NMC) global analysis and forecast system is described. Unlike conventional nonlinear normal-mode initialization, the incremental nonlinear initialization does not zero-out gravity-mode tendencies but still removes imbalances introduced by the analysis as effectively as the conventional initialization. Incremental initialization can be set up as a relatively simple modification of the conventional (Machenauer) initialization. The tests performed with the NMC global analysis-forecast system indicate that the problems pointed out for other initialization methods (i.e., undesirable changes to the forecast model guess, especially the tidal modes, sensitivity to the way diabatic tendencies are calculated, and lack of nonlinearity) are all greatly reduced by the nonlinear initialization of analysis increments. Based on these results, the new initialization was implemented into the NMC operational suite on 13 December 1989.